The present invention relates to a method and apparatus for removing undesirable condensible materials such as alkali compounds which may be contained within the material charge or the fuel burned within a shaft furnace such as a blast furnace utilized in the production of iron.
Various thermal processing methods and apparatus such as the production of cement clinker and the reduction of iron ore in a blast furnace process utilize materials which contain undesirable condensible compounds such as alkali compounds such as potassium and sodium compounds. In the processing of ores in a blast furnace these compounds may occur in the form of complex SiO.sub.2 /Al.sub.2 O.sub.3 containing materials. Undesirable compounds may also be contained in some fuels utilized in shaft furnaces. During the thermal processing of these materials, temperatures in excess of 1500.degree. F. are encountered. At these high temperatures, alkali compounds which may be contained within the charge of material or the fuel are volatilized and will rise in the furnace with spent combustion gases. As the gases rise within the furnace heating the fresh material, the gases are cooled and the volatilized compounds will condense on the fresh material. As the fresh material descends through the furnace and reaches the higher temperature, the alkali compounds within the fresh material as well as those which have previously condensed on the surface of the fresh material will volatilize. The result is a build-up in what is known as the "alkali cycle". By repeated cycles of reduction, gasification-recondensation alkali vapors are accumulated in the furnace.
The alkali build-up resulting from the alkali cycle within a furnace can, in the case of a cement making application where a suspension preheater is utilized, result in the coating of vessel walls and ducts and the eventual plugging of the ducts thereby reducing material and gas flow. This build-up of material must be removed from time to time in order to assure continuous operation of the apparatus. In addition, in a cement making operation, the alkali cycle will reach a level where the undesired compounds will no longer volatilize, but will enter the clinkering furnace with the feed material. This can result in excessive alkalis in the finished product thereby producing a product which does not meet the specifications for finished cement.
In cement making operations using a suspension preheater, apparatus may be provided for withdrawing a portion of the exhaust gases from the clinkering furnace which may be rich in volatilized alkali compounds and allowing the balance of the gases to be used in the preheater. Such a system is known as a "by-pass" and a typical system is shown in U.S. Pat. No. 3,365,521 issued Jan. 23, 1968.
In the case of a blast furnace, only a small part of the alkali in the raw materials may be reduced, gasified and rise with the furnace gases, but as the furnace gases cool, the undesirable compounds condense on the fresh feed. These condensed alkali compounds are more easily volatilized as the new feed descends in the furnace. The repeated volatilization, condensation, volatilization results in an alkali build-up in the furnace. This build-up can result in damage to the coke in the furnace and can result in plugging the porous pellets which are charged to the blast furnace. Since the open pellet porosity facilitates the flow of reducing gases through the pellet, the plugging of this porosity thereby impairing the iron reduction process. In view of this, it is currently necessary to select raw materials which do not contain excessive alkali compounds.
If the alkalis could be removed without a resulting build-up in the alkali cycle, it is believed that an iron rich, high basicity pellet with magnesium oxide such as a dolomite pellet can be utilized which may improve the blast furnace operation. The problem of alkali build-up and the advantages to be gained by the control of alkali levels is generally described in an article entitled "Pellets: The Future Blast Furnace Burden" by Per-Adrian Ilmoni at pages 6 to 8 of Skillings Mining Review, Sept. 21, 1985.
While the alkali build-up problem in conjunction with suspension preheaters has been solved by withdrawing a portion of the gases exiting the rotary kiln which gases contain volatilized alkali compounds to thereby avoid the alkali cycle build-up, such as shown in the aforesaid U.S. Pat. No. 3,365,521 it is believed that this technology has not heretofore been applied to shaft furnaces in general and specifically to blast furnaces. Prior to the present invention, two ways to control blast furnace alkali cycles are to use low alkali raw materials and or acid pellets. Basic iron ore pellets will result in too high level of alkali. By removing or "bypassing" the part or all of the alkali rich gases from the incoming feed, an alkali build up can be avoided.